How the quantities are worked out
Wet concrete shrinks when the voids fill, so the dry volume of materials is taken as 1.54 times the wet volume. That dry volume is split in the mix ratio. For M20 (1 : 1.5 : 3, total 5.5 parts) and 1 m³ of concrete: cement = 1.54 × 1 ÷ 5.5 = 0.28 m³ = 403 kg ≈ 8.06 bags; sand = 0.42 m³ (about 14.8 cft); aggregate = 0.84 m³ (about 29.7 cft).
Nominal mix ratios
M5 = 1 : 5 : 10, M7.5 = 1 : 4 : 8, M10 = 1 : 3 : 6, M15 = 1 : 2 : 4, M20 = 1 : 1.5 : 3 and M25 = 1 : 1 : 2. The number after M is the characteristic compressive strength in N/mm² (MPa) after 28 days, so M20 concrete is designed for 20 N/mm².
Nominal mix or design mix?
IS 456 permits nominal mixes for concrete up to M20. For M25 and above, a design mix based on trial batches with your actual cement, sand and aggregate is recommended; it usually uses less cement for the same strength. Treat the M25 row here as an estimate for small works.
Example: a slab
A 5 m × 4 m slab 125 mm (0.125 m) thick holds 2.5 m³ of concrete. In M20 that needs about 20.2 bags of cement, 1.05 m³ of sand and 2.1 m³ of aggregate, before wastage.